1. Field of the Invention
Embodiments of the present invention relate generally to fuel cells which utilize an external fuel source to generate electric power as a result of a chemical reaction between the fuel and an electrolyte. More particularly, embodiments of the present invention relate to novel fuel cell designs that utilize a combustion chamber manifold internal to the fuel cell stack for combusting and/or transferring fuel exhaust gases from an anode passage, or mixtures thereof with other gases, to a cathode passage. Embodiments of the present invention further relate to novel fuel cell designs that utilize a gas delivery manifold external to the fuel cell stack for delivering carrier and/or cooling gases for further use in the fuel cell system.
2. Description of Related Art
In fuel cells of conventional design having carbonate as an electrolyte and which attempt to recycle gases, carbonaceous fuels such as hydrocarbons are mixed with steam and/or air to produce hydrogen and other gases which are then provided to the fuel cell anode passage for the required anode reaction. At the cathode passage, oxidant or air and carbon dioxide are supplied for the required cathode reaction. The carbon dioxide supplied to the cathode passage is typically generated from the anode exhaust which typically includes as components steam, carbon dioxide, carbon monoxide, and hydrogen. Conventionally, the anode exhaust is directed via high temperature stainless steel tubing to a separate burner unit exterior to the fuel cell where the hydrogen and carbon monoxide are then combusted with air to produce carbon dioxide for use in the cathode reaction.
The carbon dioxide, along with other gases within the burner unit, are then directed back into the fuel cell via high temperature stainless steel tubing which is coupled to an inlet manifold of the cathode passage. The disadvantages of this type of external burner unit fuel cell system include the cost associated with the external burner unit and the high temperature stainless steel tubing along with the system being comprised of separate units and tubing which increases heat loss from the system, as well as leaks or breaks in the system thereby affecting fuel cell performance. In addition, this type of conventional fuel cell must be heated by a separate heating unit external to the fuel cell to an operating temperature typically in the range of 650.degree. C. to transform the electrolyte associated with carbonate fuel cells into a molten form before the fuel cell can generate electricity.
A second type of externally manifolded fuel cell system includes a catalytic burner unit configured directly within the cathode inlet manifold itself but exterior to the cathode gas passages. However, this fuel cell design unnecessarily increases the complexity of the cathode inlet manifold itself and suffers from the performance, economic and repair drawbacks associated with using a expensive catalyst system to effect combustion of gases.
A third type of fuel cell system is described in U.S. Pat. No. 5,422,195. In this type of fuel cell, the fuel and oxidant gases are simply mixed in the oxidant manifold or cathode gas passages. No specific combustion zone is provided so the gas will combust when and if there is sufficient energy anywhere in the system, including the interior of the cathode passages which may create undesirable temperature gradients with the fuel cell stack.
A fourth type of fuel cell such as that disclosed in EP 081 0684A2 transfers the spent fuel directly to the oxidant by small holes or pores in the electrolyte matrix structure. This fuel cell is disadvantageous in that unused fuel must pass through the cathode on the way to the oxidant gas chamber allowing unreacted and undiluted hydrogen gas to react with the NiO cathode according to the reaction: H.sub.2 +NiO=Ni+H.sub.2 0. This reduction of the cathode to metallic Ni at these locations may cause cathode degradation and shrinkage thereby reducing its ability to support the matrix/electrolyte structure above and leading to cracking and further leakage and degradation of the cell. The fuel cell is further disadvantageous in that the concentration of H.sub.2 decreases as the gas is reacted at the anode and passes from the fuel inlet to the fuel exit side of the cell. Therefore the concentration of unreacted H.sub.2 fuel will be higher in the passages through the electrolyte structure which are closer to the fuel inlet. This fuel, therefore, will not generate electricity thereby reducing efficiency of the fuel cell as a whole.